Non-toxic immune stimulating enterotoxin compositions

ABSTRACT

Pyrogenic toxins, such as staphylococcal enterotoxins, modified in the disulfide loop region are provided. The modified toxins retain useful biological properties but have substantially reduced toxicity compared to the corresponding unmodified native toxin. The native pyrogenic toxins are typically modified by deletions within the disulfide loop region to produce modified enterotoxins having 100-fold or greater decrease in toxicity.

GOVERNMENT SUPPORT

This invention was funded in part by the United States Department of Health under NIH grant R11A128401. The United States Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

A group of biological agents termed superantigens has been described based on their ability to stimulate monocytes and unprimed CD4⁺ (MHC Class II restricted) and CD8⁺ (MHC Class I restricted) T-cells. Included with the designation of superantigens are the enterotoxins of Staphylococcus aureus.

The enterotoxins of Staphylococcus aureus form a group of serologically distinct proteins, originally designated A, B, C₁, C₂, C₃, D, E, G, and H. Subsequently, a number of variants have been described. These proteins and toxic shock syndrome proteins were originally recognized as the causative agents of staphylococcal food poisoning. Ingestion of preformed enterotoxin in contaminated food leads to the rapid development (within two to six hours) of symptoms of vomiting and diarrhea that are characteristic of staphylococcal food poisoning. Toxic shock syndrome toxin-1, TSST-1, a distantly related protein also produced by S. aureus, is classically responsible for the toxic shock syndrome, although other staphylococcal enterotoxins may result in the syndrome due to the induction of cytokines.

Other agents which have been identified as superantigens include for example, the staphylococcal exfoliative toxins, A and B; the mammary tumor virus supefantigen; rabies virus nucleocapsid protein; pyrogenic exotoxins A, B, C from S. pyrogens; and the Mycoplasma arthritides mitogen. Additional biological agents which have been demonstrated to have superantigen properties include the human immunodeficiency virus (HIV), gp120 and peptides from HIV, mouse mammary tumor virus and feline immunodeficiency virus. A Leishmania peptide antigen has also been disclosed as a superantigen.

Superantigens, unlike conventional antigens, do not require processing in-vivo. In general, superantigens have two binding regions, one of which interacts with the Class II major histocompatibility complex (MHC) on the antigen presenting cell and the other which interacts with the VP variable region of the T-cell receptor on CD4 and/or CD8 cells. Various enterotoxins bind to one or more of the different Vβ receptor epitopes. In contrast to conventional antigens, superantigens do not occupy the T-cells receptor cleft but are felt to bind to an external region thus explaining the ability to activate a broad population of T-cells.

Enterotoxins produced by Staphylococcus aureus include a group of related proteins of about 20 to 30 Kd. The complete amino acid composition of a number of staphylococcal enterotoxins and streptococcal pyrogenic exotoxin has been reported (see e.g., PCT Patent Appl. No. WO 93/24136.)

Staphylococcal enterotoxins (“SEs”) were initially classified on the basis of their antigenic properties into groups A, B, C1, C2, C3, D, and E. Subsequent relatedness was based on peptide and DNA sequence data. Among the staphylococcal enterotoxins, groups B and C are closely related and groups A, D, and E are closely related in amino acid sequence. SEC1, SEC2, and SEC3 and related isolates share approximately 95% sequence similarity. Table 1 shows the alignment of the predicted sequences of the eight known SEC variants following cleavage of the signal peptide. The N-terminus of each of the mature proteins was verified by amino acid sequencing. Amino acid positions that contain residues that are not conserved among all SEC are indicated by asterisks. SEB and SEC are approximately 45-50% homologous. In contrast, non-enterotoxin superantigens, TSST-1 and Streptococcal Pyrogenic Enterotoxin C(SPEC) share only approximately 20% primary sequence homology to SEC. Despite these differences, the tertiary structure of the various enterotoxins show nearly identical folds.

The staphylococcal enterotoxins A, B, C₁, C₂, C₃, D, E, G and H share a common structural feature of a disulfide bond not present in other enterotoxins. Table 2 shows the position of the disulfide bond in a number of enterotoxins. Data in reference to the active sites of the enterotoxin molecule in relationship to biological activity, MHC binding, and TCR binding has been obtained. Sequence data demonstrate a high degree of similarity in four regions of the enterotoxins (See Table 3). The peptides implicated in potential receptor binding correspond to regions 1 and 3 which form a groove in the molecule. Amino acid residues within and adjacent to the α₃ cavity of SEC3 have been shown to relate to T-cell activation. TABLE 2 LOCATION OF DISULFIDE LOOP IN STAPHYLOCOCCUS ENTEROTOXINS AMINO ACID AMINO ACID SEQUENCE OF ENTEROTOXIN RESIDUES DISULFIDE LOOP SEA 96-106 96→CAGGTPNKTAC SEB 93-114 93→CYFSKKTNDINSHQTPKRKTC SEC1 93-110 93→CYFSSKDNVGKVTGGKTC SEC2 93-110 93→CYFSSKDNVGKVTGGKTC SEC3 FRI 913 93-110 93→CYFSSKDNVGKVTGGKTC SEC3 FRI 909 93-110 93→CYFSSKDNVGKVTSGKTC SEC 4446 93-110 93→CYFSSKDNVGKVTGGKTC SEC-Bovine 93-110 93→CYFSSKDNVGKVTGGKTC SEC-Ovine 93-110 93→CCFSSKDNVGKVTGGKTC

The staphylococcal enterotoxins are potent activators of T-cells, resulting in proliferation and the generation of cytotoxic T-cells. SEA is a potent T-cell mitogen eliciting strong polyclonal activation at concentrations of 10⁻¹³ to 10⁻¹⁰ molar in human systems.

The staphylococcal enterotoxins, aside from the acute gastroenteritis and toxic shock syndrome associated with them, have been shown to have a variety of other beneficial biological effects. The biological effects of these agents and the toxic shock syndrome are due in part to the ability of staphylococcal enterotoxins to induce cytokines. Various cytokines described include IL-1, IL-2, and tumor necrosis factor (“TNF”). More recently SEB and toxic shock syndrome toxin (“TSST-1”) have been shown to induce interleukin-12, an inducer of cell mediated immunity, in human peripheral blood mononuclear cells. (See Leung et al., J Exp Med, 181:747 (1995)). The antitumor activity of treating cancer in rabbits utilizing 40 to 60 μg/kg of a staphylococcal enterotoxin has been disclosed in PCT Patent Appl. Nos. WO 91/10680 and WO 93/24136.

Exposure to enterotoxin either in-vitro or in-vivo leads to depletion of T-cells having the appropriate Vβ receptor through programmed cell death in some strains of mice, specifically Balb/c and CBA/2. Cell death can be prevented by high doses of retinol or RU-38486. Programmed cell death has not been observed upon exposure of human cells to enterotoxins.

Although the systemic lethal toxicity of enterotoxins has been related to their ability to induce cytokines, particularly IL-1, IL-2 and gamma interferon, lethal toxicity also appears to be related to a synergistic activity with endogenous endotoxins and the ability of the liver to detoxify endotoxins. Although a number of animals have been utilized to evaluate lethality, the accepted model is the continous infusion over a period of time, usually 4 days, in rabbits. The direct toxic dose varies among various species. The 50% lethal dose of TSST-1 is approximately 50 μg/kg for Balb/c mice. Piglets, although showing clinical manifestations of toxic shock syndrome, tolerate doses of 100 μg/kg of TSST-1. TSST-ovine is known to be non-toxic at doses of 200 μg in rabbits.

In Dutch belted rabbits, intramuscular injection of 50 mg/kg of staphylococcal enterotoxin B caused death. Intravenous injection at 0.5 to 1.0 mg/kg of enterotoxin A or B in rhesus macaques results in hypotension and death (Liu C. T., et al Amer J Vet Res 39:279 and 1213, 1978).

In contrast to other species, man is extremely sensitive to enterotoxins. One (1) mg of TSST-1, approximately 15 nanogram/kg, can be lethal for man. Therefore, the recommended doses currently proposed in the art for treating man are unacceptable. There is a need, therefore, for mutant staphylococcal enterotoxins which are non-toxic at anticipated doses for man while still retaining desirable biological activity.

Several studies of staphylococcal enterotoxin have identified a number of biologically active modified or mutant enterotoxins with reduced toxicity. Carboxymethylation of SEB results in a loss of gastrointestinal toxicity but not mitogenic activity. Studies with the TSST-1 have demonstrated the active site to be between amino acids residue 115 and 141. Point mutation of site 135 from histidine to alanine results in a loss of mitogenic activity and toxicity (See Bonventre P. F., et al. Infect Immun 63:509 (1995)). Studies with the staphylococcal enterotoxin SEC1 demonstrated that the disulfide bond between residue 93 and 110 is not required for activity (See Hovde et al., Mol Microbiol 13:897 (1994)). Studies of the molecular binding region of staphylococcal enterotoxin B using overlapping peptides demonstrated peptide 124 to 154 inhibited SEB induced mitogenic activity.

Based on the known biological activities of the toxic native enterotoxins, it is desirable to create mutants which are at least 1000-fold or more less toxic compared to native enterotoxins and retain biological activity. Recent studies have demonstrated that mutant enterotoxins can be produced which retain certain biological activities and which may be significantly less lethal as determined in rabbits. A mutant of the TSST-1 enterotoxin which differs in amino acid 136 and is non-lethal at ten times the lethal dose of the native toxin (in rabbits), but retains biological activity has been disclosed. A number of mutants of SEC1, unable to form a disulfide bond, have been reported to be ten times less toxic than the native toxin while retaining biological activity. (See e.g., Hovde et al., Molec Microbiol 13:897 (1994)).

SUMMARY OF THE INVENTION

The present invention relates to modified versions of disulfide loop-containing bacterial pyrogenic toxins. The modified pyrogenic toxins retain useful biological properties but have substantially reduced toxicity (e.g., toxicity reduced by at least about 10-fold) compared to the corresponding unmodified native toxin. Selected deletions within the disulfide loop region can produce modified toxins having a 100-fold or greater decrease in toxicity. The toxicity of the modified toxin can be measured based on a variety of parameters, including emetic response inducing activity, fever inducing activity, and lethality (as measured by LD₅₀ in Dutch Belted rabbits).

Examples of the present modified toxins include disulfide loop region deletion mutants of native toxins derived from Staphylococcus aureus or Streptococcus pyrogenes. Suitable native disulfide loop-containing toxins which may be modified according to the present invention include Type A, B, C, D and E staphylococcal enterotoxins as well as streptococcal pyrogenic enterotoxin A (“SPEA”) and streptococcal superantigen (“SSA”) produced by S. pyrogenes.

The pyrogenic toxins constitute a family of exotoxins produced by species of gram positive cocci, such as Staphylococcus and Streptococcus. The pyrogenic toxins are characterized by shared ability to induce fever, enhance host susceptibility to endotoxin shock, and induce T cell proliferation through action as superantigens. Examples of pyrogenic toxins include TSST-1, staphylococcal enterotoxins (SEs), and streptococcal pyrogenic exotoxins (SPEs). In addition to the activities listed above, some pyrogenic toxins have additional activities that are not shared by all pyrogenic toxins. For example, the staphylococcal enterotoxins induce emesis and diarrhea when ingested. Structurally, the pyrogenic toxins have varying degrees of relatedness at the amino acid and nucleotide sequence levels. A number of the pyrogenic toxins include a disulfide loop as a structural feature. The staphylococcal enterotoxins have a disulfide loop, as do some others in this family. Examples of other pyrogenic toxins that have a disulfide loop are the streptococcal superantigen (“SSA”) and streptococcal pyrogenic exotoxin A (“SPEA”).

The pyrogenic toxins have varying degrees of relatedness which provides the basis for separating some of them informally into subgroups. One subgroup includes staphylococcal type B and C enterotoxins (“SEB” and “SEC”), as well as SPEA and SSA. These toxins share between about 49% to greater than 95% amino acid sequence homology (Reda et al, Infect. Immun., 62:1867-1874: (1994)). Another subgroup of related pyrogenic toxins include staphylococcal type A and E enterotoxins (SEA and SEE) which are 83% homologous to each other (Couch et al, J. Bacteriol., 70:2954-2060 (19______), less so but significantly to SED (Bayles et al., J. Bacteriol., 171:4799-4806 (1989)). The amino acid sequences of this second subgroup is more distantly related to SEB, SEC, SPEA, and SSA. Examples of pyrogenic toxins having disulfide bonds are present in both of these two subgroups. TSST-1 and streptococcal pyrogenic exotoxins B and C(SPEB and SPEC) are examples of a third subgroup of less related toxins. Although toxins from this third subgroup may share some conserved regions (see table 3) with toxins from the other subgroups, there is little overall sequence homology between toxins in the third subgroup and the pyrogenic toxins in the other two subgroups. Neither TSST-1, SPEB nor SPEC includes a disulfide loop.

The disulfide loop region of a native pyrogenic toxin, such as a native staphylococcal enterotoxin, is generally modified through deletion of a number of amino acid residues within the loop. The modification typically includes deletion of amino acid residues within the disulfide loop region and may include one or more substitutions and/or additions to the remaining loop residues. After modification, the disulfide loop region typically contains no more than about 10 and, preferably, no more than about 6 amino acids residues. In another embodiment of the invention, a modified pyrogenic toxin is formed from a native pyrogenic toxin modified by deletion of at least 40% of the amino acid residues within the disulfide loop region, e.g., by deletion of 8 or more amino acid residues from the disulfide loop region of a native type C staphylococcal enterotoxin.

The present invention is also directed to isolated nucleic acids which include a nucleotide sequence encoding a modified pyrogenic toxin.

DETAILED DESCRIPTION OF THE INVENTION

The current invention relates to modified versions of pyrogenic toxins, such as staphylococcal enterotoxins produced by modifications which include deletions in the disulfide bond region. The present invention describes the feasibility of obtaining mutant pyrogenic toxins which retain biological activity but demonstrate significantly lower toxicity at doses well in excess of the normal lethal dose and levels anticipated for human therapeutic use.

The modified pyrogenic toxins are derived from a native pyrogenic toxin having a disulfide loop. The terms “disulfide loop” and “disulfide loop region” are used interchangeably herein. As employed in this application, these terms refer to the sequence of about 10 to about 30 amino acid residues forming a loop defined by a disulfide bond in a native pyrogenic toxin. The term “disulfide loop region” also refers to the corresponding portion of the sequence of a modified pyrogenic toxin which has been produced by deletion, substitution or addition of one or more amino acid residues of the disulfide loop of a native pyrogenic toxin. The disulfide loop region is defined to begin with the N-terminal Cys residue and end with the C-terminal Cys residue of the loop, e.g., amino acid residues 93-110 of staphylococcal enterotoxin C1. As used herein, the positions of the disulfide loop region for a given native pyrogenic toxin are numbered beginning with the N-terminal cyteine residue in the loop, e.g., position 93 of type B or C staphylococcal enterotoxins is also referred to herein as position 1 of the disulfide loop region.

The modification of the disulfide loop typically includes deletion of at least about 40% of the amino acid residues within the disulfide loop. For example, this generally results in the deletion of at least about 8 amino acid residues from the disulfide loop region of an SEC. Examples of native staphylococcal enterotoxin which can be modified to form the present low toxicity toxins include type A, B, C, D, E, G, and H staphylococcal enterotoxins. Type C staphylococcal enterotoxins such as staphylococcal enterotoxin C1, staphylococcal enterotoxin C2, staphylococcal enterotoxin C2, staphylococcal enterotoxin C-MNCopeland, staphylococcal enterotoxin C-4446, staphylococcal enterotoxin C-bovine (GenBank Accession No. L13374), staphylococcal enterotoxin C-canine (GenBank Accession No. V 19526) and staphylococcal enterotoxin C-ovine (GenBank Accession No. L13379) are particularly suitable enterotoxins for modification by deletion of a portion of the disulfide loop region to form a staphylococcal enterotoxin with decreased toxicity.

The modified disulfide loop region generally contains no more than about 10 amino acid residues and, preferably no more than 6 amino acid residues. For example, a type C staphylococcal enterotoxin, such as staphylococcal enterotoxin C1, can be modified to delete amino acid residues 98-106 (residues 6-14 of the disulfide loop) to form a modified staphylococcal enterotoxin having substantially reduced toxicity. An even greater reduction in toxicity is produced by deleting amino acid residues 95-106 (disulfide loop residues 3-14) of staphylococcal enterotoxin C1. Both of these mutants, despite having substantially reduced toxicity, are biologically active as evidenced by their ability to stimulate the uptake of thymidine by human peripheral blood mononuclear cells.

In addition to deletion of a number of the disulfide loop residues, substitution of a cysteine residue for the residue at position 2 of disulfide loop can contribute to a decrease in the toxicity of a pyrogenic toxin such as a type C staphylococcal enterotoxin. In a preferred embodiment, staphylococcal enterotoxin C1 can be modified so that in addition to deletion of a substantial portion of the residues in the loop region, the amino acid residue at position 2 of the loop, Tyr-94, is replaced by a cysteine.

Other examples of preferred embodiments of the invention include modified staphylococcal enterotoxins having no more than 10 amino acid residues and, preferably, no more than 6 amino acid residues in the disulfide loop region. Particularly preferred examples include staphylococcal enterotoxins with no more than about 9 amino acid residues which include the sequence Cys-Gly-Lys-Thr. One example of such a mutant is staphylococcal enterotoxin C1 modified to have the disulfide loop region sequence Cys-Cys-Gly-Lys-Thr-Cys.

The methods of this invention employed in preparing mutant enterotoxins, and their screening, analysis, and purification are known in the art and described herein. Site directed mutagenesis can be carried out by initially cloning the SECI structural gene SECMNDON on a 1.4 Kb MindITI-BamH1 restriction fragment. A unique SphI restriction site (5′-GCATGC-3′) can be introduced into SEC_(MNDON) in the region coding for the disulfide loop at nucleotides 301-306 (5′-GTAGGT-3′) using a commercially available kit (Altered Sites in vitro Mutagenesis System, Promega). Potential mutants may be screened by Sph1 digestion and confirmed by nucleic acid sequence analysis.

Select SEC_(MNDON) deletion mutants were cloned into an E. coli cell line using pMIN164 an E. coli-S. aureus shuttle vector. To facilitate protein purification, recombinant plasmids from E. coli RR1 transformants were transformed into S. aureus RN4220 by standard protoplast transformation techniques. Plasmids containing cloned toxin genes were maintained in S. aureus RN4220 under erythromycin (50 mg/ml) selection. For purification of native and mutant derivatives of the toxin recombinants, S. aureus RN4220 cultures were grown in dialyzable beef heart media supplemented with 1% glucose buffer (330 mM glucose; 475 mM NaHCO₃; 680 mM NaCl; 137 mM Na₂HPO₄.H₂O; 28 mM L-glutamine) and erythromycin (50 mg/ml) followed by ethanol precipitation.

Purification of ethanol precipitated proteins was accomplished by preparative flat bed isoelectric focusing (IEF). Following the IEF run, proteins in select fractions were pooled, dialyzed to remove the amphylotes and then visually assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

The toxicity and biological activity of the mutants was evaluated using standard methods known to those skilled in the art as described below. The emetic activity of SEC 1, and the SEC 1 mutant toxins was determined by a modification of the standard monkey feeding assay using young adult pigtail monkeys (M. nemestrina). The animals were manually restrained while the toxin was administered through a nasogastric tube (Infant feeding tube; Becton Dickinson, Rutherford, N.J.). Toxins were screened for retention of emetic activity at a dose of 10 μg/Kg which is approximately 100 times the minimal emetic dose for SEC 1. Non-emetic toxins were tested for residual emetic activity at a high dose of 250 μg/Kg.

The mitogenic capacities of mutant toxins were compared to that of SECI native toxin by using human peripheral blood mononuclear cells (PMBC) in a standard 4-day assay. Solutions of native and mutant toxin were added in triplicate to PBMC cell suspensions, 1×10⁶ cells/ml, in 96-well tissue culture plates. This mixture of cells and toxin was then incubated at 37° C. under atmospheric conditions of 6% CO₂ for 72 hours. [³H]-thymidine (New Research Products, Boston, Mass.) at a concentration of 1 μCi/25 μl in a complete RPMI medium was added to each well and allowed to incubate under the same conditions for an additional 18-24 hours. After incubation, radiolabeled cellular nucleic acids were harvested onto glass fiber filters (Skatron, Sterling, Va.) using a semi-automatic cell harvester (Skatron). Lymphocyte proliferation was quantitated by measuring incorporation of [³H]-thymidine into cellular DNA using a liquid scintillation counter (TR1-CARB 1500 Liquid Scintillation Center, Packard, Rockville, Md.).

The ability of a staphylococcal enterotoxin to induce a fever response and enhance susceptibility of lethal endotoxic shock can be determined in vivo using a standard rabbit model (Bohach et al., Infect. Immun., 55, 428 (1987)). Following conditioning in a test rack and having baseline body temperature recorded, adult New Zealand White rabbits can be initially intravenously injected with a native or mutant toxin at a concentration of 10 μg/kg in sterile physiological saline. Sterile saline and purified SEC1 toxin are typically used as negative and positive controls respectively. Following toxin injection, rabbit body temperature is generally monitored rectally every hour for four hours. Four hours after initial treatment, an intravenous injection of lipopolysaccharide (“LPS”) from Salmonella typhimurium (Difco Laboratories, Detroit, Mich.) is administered intravenously at a concentration of 10 μg/Kg in sterile saline. Animals are then observed for signs of shock and mortality for 48 hours after LPS injection.

Cytokine induction may be determined by utilizing isolated mononuclear cells from heparinized venous blood. Briefly, heparinized venous blood is obtained and layered onto lymphocyte separation medium. The tubes are spun and the mononuclear layer is harvested, washed in PBS, resuspended in RPMI containing 10% FCS and adjusted to 1×10⁶ cells per ml. Aliquots of 100 μl are typically placed into 96-well microtiter plates. The enterotoxins were added in 100 μl to a final concentration of 1 ng/ml. Following incubation at 37° C. for 48-72 hours, the supernatant was harvested can be assayed for cytokines using commercially available kits from R&D Systems Minneapolis, Minn.

The invention will be further described by reference to the following examples. These examples illustrate but do not limit the scope of the invention that has been set forth herein. Variation within the concepts of the invention will be apparent.

EXAMPLE 1

The SEC 1 structural gene, SECMNDov, was previously cloned on a 1.4 Kb HindIII-BamH1 restriction fragment (Bohach et al., Infect. Immun., 55, 428 (1987)). A unique SphI restriction site (5′-GCATGC-3′) was introduced into SEC_(MNDON) in the region coding for the disulfide loop at nucleotides 301-306 (5′-GTAGGT-3′) using a commercially available kit (Altered Sites in vitro Mutagenesis System, Promega). Potential mutants were screened by Sph1 digestion and confirmed by nucleic acid sequence analysis.

EXAMPLE 2

The unique Sph1 site was used to linearize the SEC_(MNDON) gene so that bi-directional deletions could be generated using Bal-31 exonuclease (Boehringer Mannheim, Indianapolis, Ind.). Bal-31 generated deletion mutant plasmids were ligated and transformed into E. coli TG1. Transformants growing on LB-AMP (125 μg/ml) with in-frame stable deletions were detected by screening with SEC1 specific rabbit antisera in immunodiffusion and analyzed by nucleic acid sequence analysis. Sequencing reactions were done using Sequenase Version 2.0, a commercially available kit (U.S. Biochemical Corp., Cleveland, Ohio).

Three deletion mutants were chosen for detailed analysis based on the size and location of the deletions in the loop region of the toxin; SEC1-4AA, SCE1-9AA, and SEC1-12AA. Following purification, each of the disulfide loop mutant toxins could be distinguished from wild type SEC1 on the basis of size when analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis.

The nucleic acid and amino acid sequences for SEC1 and three deletion mutants are shown in Table 4.

EXAMPLE 3 Mutant Toxin Purification

Select SEC_(MNDON) deletion mutants were cloned into E. coli RR1 using pMIN164, an E. coli-S. aureus shuttle vector. To facilitate protein purification, recombinant plasmids from E. coli RR1 transformants were transformed into S. aureus RN4220 by standard protoplast transformation techniques. Plasmids containing cloned toxin genes were maintained in S. aureus RN4220 under erythromycin (50 μg/ml) selection. For purification of the recombinantly produced native and mutant derivatives of the toxin, S. aureus RN4220 cultures were grown in dialyzable beef heart media supplemented with 1% glucose buffer (330 mM glucose; 475 mM NaHCO₃; 680 mM NaCl; 137 mM Na₂HPO₄.H₂O; 28 mM L-glutamine) and erythromycin (50 μg/ml) followed by ethanol precipitation.

Purification of ethanol precipitated proteins was accomplished by preparative flat bed isoelectric focusing (“IEF”). Following the IEF run, proteins in select fractions were pooled, dialyzed to remove the ampholytes and then visually assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

EXAMPLE 4 Emesis Assay

Assays of the emetic activity of SEC1 and the modified SEC1 toxins were conducted using a modification of the standard monkey feeding assay using young adult pigtail monkeys (M. nemestrina). The animals were manually restrained while the toxin was administered through a nasogastric tube (Infant feeding tube; Becton Dickinson, Rutherford, N.J.). Toxins were screened for retention of emetic activity at a dose of 10 μg/Kg which is approximately 100 times the minimal emetic dose for SEC 1. Non-emetic toxins were tested for residual emetic activity at a high dose of 250 mg/Kg.

It has been previously shown that the minimal emetic dose of wild type SEC1 for M. nemestrina was 0.1 μg/Kg. For initial experiments in which the emetic ability was tested, loop mutant toxins were administered at 10 μg/Kg. This insured an excess of toxin over the wild type SEC1 minimal emetic dose. Following intragastric toxin inhibition, animals were observed for at least 12 hours for an emesis response. SEC1-12AA did not show emesis at the 10 μg/Kg concentration and was subsequently tested for emesis at a higher toxin concentration, 250 μg/Kg. Even at this higher doseage level, the SEC1-12AA loop mutant toxin showed no emetic response (See Table 5). TABLE 5 Emetic Response^(c) of SEC1 Loop Deletion Mutants Dose^(a) SEC1 SEC1-4aa SEC1-9aa SEC1-12aa 250 μg/Kg ND^(b) ND ND 0/2  10 μg/Kg 2/2 2/2 2/2 0/2  1 μg/Kg 2/2 2/2 0/1 ND ^(a)μg Toxin/Kg Body Weight ^(b)ND = Not Determined ^(c)Number animals exhibiting emetic response/Total number of animals

EXAMPLE 5 Pyrogenicity and Enhancement of Lethal Endotoxic Shock

The SEC1 mutant toxin's ability to induce a fever response and enhance susceptibility of lethal endotoxic shock was determined in vivo using a previously described rabbit model. Following 1 hour of conditioning in a test rack and having baseline body temperature recorded, adult New Zealand White rabbits were initially intravenously injected with SEC1 or SEC1 mutant toxin at a concentration of 10 μg/Kg in sterile physiological saline. Sterile saline and purified SEC1 toxin were used as negative and positive controls, respectively. Following toxin injection, rabbit body temperature was monitored rectally every hour for four hours. Four hours after initial treatment an intravenous injection of lipopolysaccharide (LPS) from Salmonella typhimurium (Difco Laboratories, Detroit, Mich.) was administered intravenously at a concentration of 10 μg/Kg in sterile saline. Animals were watched for signs of shock and mortality for 48 hours after LPS injection.

Wild type SEC1, administered at both 10 μg/Kg and 1 μg/Kg, showed a typical temperature rise (Table 6) as well as an enhanced susceptibility to endotoxic shock (Table 7). TABLE 6 Maximum Temperature Rise in ° C. in Rabbits Given Noted Amount of Native SEC1, Compound to SEC1-4, SEC1-9, or SEC1-12 Mutant SEC1 SEC1-4aa SEC1-9aa SEC1-12aa  100 μg ND ND ND 0.6   10 μg 1.6 1.46 0.9 0.45   1 μg 1.3 1 0.43 ND  0.1 μg 1.05 0.65 ND ND 0.01 μg 0.475 0.425 ND ND

TABLE 7 Lethality in Dutch Belted Rabbits of Native SEC1 and SEC1 Deletion Mutants Dose (μg) SEC1 SEC1-4 SEC1-9 SEC1-12 100 ND ND ND 0/3 10 3/3 3/3 2/3 0/3 1 2/2 3/3 0/3 ND 0.1 3/3 2/3 ND ND 0.01 1/4 0/3 ND ND

SEC1-12AA loop mutant toxin concentration was increased to 100 μg/Kg after test animals showed both reduced pyrogenic effects and no susceptibility to endotoxin shock at the initial dose tested, 10 μg/Kg. Following this log fold increase of the SEC-12AA mutant toxin to 100 μg/Kg it was seen that the mutant toxin induced a slight temperature increase over the initial SEC1-12AA loop mutant toxin concentration tested but was not lethal in the assay.

EXAMPLE 6 Mitogenicity

The mitogenic capacity of mutant toxin was compared to that of SEC1 native toxin by using human peripheral blood mononuclear cells (PMBC) in a standard 4-day assay (Bohach et al., Infect. Immun., 55, 428 (1987)). Human peripheral blood mononuclear cells were isolated from 30 to 60 ml of heparinized blood obtained by venipuncture. The blood was mixed 1:1 with Dulbecco's Phosphate Buffered Saline (PBS) (without Calcium or magnesium) and layered onto Fico/Lite (density 1.079 g/ml, Atlanta Biologicals, Norcross, Ga.) and centrifuged at 400×g for 20 minutes. The interface containing the mononuclear cells was removed and washed 3 times with PBS. After the last wash, the pellet was resuspended in Hank's Balanced Salt solution without calcium or magnesium and layered on Fetal Bovine Serum (Atlanta Biologicals, Norcross, Ga.) and centrifuged at 100×g for 10 minutes to remove the platelets. The cells were washed in Hank's Balanced Salt solution, Cat. No. M1211-021-LV (Atasca, Ill.). A cell count was done by Trypan Blue exclusion.

Solutions of native and mutant toxins were added in triplicate to PMBC cell suspensions, 1×10⁶ cells/ml, in 96 well tissue culture plates. This mixture of cells and toxin was then incubated at 37° C. under atmospheric conditions at 6% CO₂ for 72 hours. [³H]-thymidine (New Research Products, Boston, Mass.) at a concentration of 1 μCi/25 μl in a complete RPMI medium was added to each well and allowed to incubate under the same conditions for an additional 18-24 hours. After incubation, radiolabeled cells were harvested onto glass fiber filters (Skatron, Sterling, Va.) using a semi-automatic cell harvester (Skatron). Lymphocyte proliferation was quantitated by measuring incorporation of [³H]-thymidine into cellular DNA using a liquid scintillation counter (TI-CARB 1500 Liquid Scintillation Counter, Packard, Rockville, Md.).

Using SEC1 wild type toxin as a control it was determined that the SEC1-12AA loop mutant toxin showed mitogenic activity at a reduced effectiveness compared to native SEC1 and the SEC1-4 and SEC1-9 mutants in stimulating PMBC cells (see Table 8). TABLE 8 SEC1 Loop Mutant Mitogeneicity [³H]-Thymidine Uptake (in CPM) of Enterotoxin Stimulated Human Peripheral Blood Mononuclear Cels Dose SEC1 SEC1-4AA SEC1-9AA SEC1-12AA 0.1 pg 1918 2067 1067 978 1 pg 8533 7453 2360 954 0.01 ng 14138 15495 7682 1200 0.1 ng 21557 23689 21481 6500 1 ng 30329 28892 28951 9400 0.01 μg 33089 31640 23333 2700

All publications and patent applications in this specification are indicative of the level of ordinary skill in the art to which this invention pertains and are hereby incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated by reference.

The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention. TABLE 1 Amino Acid Sequence of Selected Staphylococcal Enterotoxins                                       40       *  *   * *    * *   *            ↓*            *  * *             *  * SEC1 1 ESQPDPTPDELHKASKFTGLMENMKVLYDDHYVSATKVKSVDKFLAHDLIYNISDKKLKNYDKVKTELLNEGLAKKYKDE 80 SEC2 1 ESQPDPTPDELHKSSEFTGTMGNMKYLYDDHYVSATKVMSVDKFLAHDLIYNISDKKLKNYDKVKTELLNEDLAKKYKDE 80 SEC3-FRI913 1 ESQPDPMPDDLHKSSEFTGTMGNMKYLYDDHYVSATKVKSVDKFLAHDLIYNISDKKLKNYDKVKTELLNEDLAKKYKDE 80 SEC3-FRI909 1 ESQPDPMPDDLHKSSEFTGTMGNMKYLYDDHYVSATKVKSVDKFLAHDLIYNINDKKLNNYDKVKTELLNEDLANKYKDE 80 SEC-MNCopeland 1 ESQPDPTPDELHKSSEFTGTMGNMKYLYDDHYVSATKVKSVDKFLAHDLIYNISDKKLKNYDKVKTELLNEDLAKKYKDE 80 SEC-4446 1 ESQPDPMPDDLHKSSEFTGTMGNMKYLYDDHYVSATKVKSVDKFLAHDLIYNISDKRLKNYDKVKTELLNEDLAKKYKDE 80 SEC-bovine 1 ESQPDPTPDELHKASKFTGLMENMKVLYDDRYVSATKVKSVDKFLAHDLIYNISDKKLKNYDKVKTELLNEDLAKKYKDE 80 SEC-ovine 1 ESQPDPTPDELHKASKFTGLMENMKVLYDDRYVSATKVKSVDKFLAHDLIYNISDKKLKNYDKVKTELLNEDLAKKYKDE 80                                       120               *            *          ↓       *      * SEC1 81 VVDVYGSNYYVNCYFSSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLIRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC2 81 VVDVYGSNYYVNCYFSSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLIRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC3-FRI913 81 VVDVYGSNYYVNCYFSSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLVRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC3-FRI909 81 VVDVYGSNYYVNCYFSSKDNVGKVTSGKTCMYGGITKHEGNHFDNGNLQNVLIRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC-MNCopeland 81 VVDVYGSNYYVNCYFSSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLIRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC-4446 81 VVDVYGSNYYVNCYFSSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLVRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC-bovine 81 VVDVYGSNYYVNCYFFSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLIRVYENKRNTISFEVQTDKKSVTAQELD 160 SEC-ovine 81 VVDVYGSNYYVNCCFFSKDNVGKVTGGKTCMYGGITKHEGNHFDNGNLQNVLIRVYENKRNTISFEVQTDKKSVTAQELD 160                                           200         *                      *        ↓               * *   *    * SEC1 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFWYDMMPAPGDKFDQSKYLMMYNDNKTVDSKSVKIEVHLTTKNGX 240 SEC2 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFQYDMMPAPGDKFDQSKYLMMYNDNKTVDSKSVKIEVHLTTKNGX 240 SEC3-FRI913 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFQYDMMPAPGDKFDQSKYLMMYNDNKTVDSKSVKIEVHLTTKNGX 240 SEC3-FRI909 161 IKARNFLINKKNLYEFNSSPYETGYIKFIESNGNTFWYDMMPAPGDKFDQSKYLMMYNDNKMVDSKSVKIEVHLTTKNGX 240 SEC-MNCopeland 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFQYDMMPAPGDKFDQSKYLMMYNDNKTVDSKRVKIEVHLTTKNGX 240 SEC-4446 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFQYDMMPAPGDKFDQSKYLMMYNDNKTVDSKRVKIEVHLTTKNGX 240 SEC-bovine 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFQYDMMPAPGDKFDQSKYLMMYNDNKTVDSKRVKIEVHLTTKNGX 240 SEC-ovine 161 IKARNFLINKKNLYEFNSSPYETGYIKFIENNGNTFQYDMMPAPGDKFDQSKYLMMYNDNKTVDSKRVKIEVHLTTKNGX 240

TABLE 3A Conserved Regions of Enterotoxin Molecules* Residue Residue Toxin # Region 1 Toxin # Region 3 SEA 79 K Y K G K K V D L Y G SEA 147 K K N V T V Q E L D L Q A R R Y L SEB 76 K Y K D K Y V D V F G SEB 152 K K K V T A Q E L D Y L T R H Y L SEC1 76 K Y K D E V V D V Y G SEC1 151 K K S V T A Q E L D I K A R N F L SEC2 76 K Y K D E V V D V Y G SEC2 151 K K S V T A Q E L D I K A R N F L SEC3 76 K Y K D E V V D V Y G SEC3 151 K K S V T A Q E L D I K A R N F L SED 74 H F K S K N V D V Y P SED 142 K K N V T V Q E L D A Q A R R Y L SEE 76 K Y K G K K V D L Y G SEE 144 K K E V T V Q E L D L Q A R H Y L SPEA 70 L F K D K N V D I Y G SPEA 137 K K M V T A Q E L D Y K V R K Y L SPEC 63 F K R D D H V D V F G SPEC 124 K D I V T F Q E I D F K I R K Y L TSST-1 56 F T K G E K V D L N T TSST-1 121 K K - - - - Q - L - I TSST-1 129                 L D F E I R H Q L *From Hoffmann et al., Infect Immunol 62: 3396 (1994).

TABLE 3B Conserved Regions of Enterotoxin Molecules (Cont.)* Residue Residue Toxin # Region 2 Toxin # Region 4 SEA 106 C M Y G G V I L H D N N SEA 209 L L R I Y R D N K T I N S E SEB 113 C M Y G G V T E H N G N SEB 213 Y L M M Y N D N K M V D S K SEC1 110 C M Y G G I T K H E G N SEC1 213 Y L M M Y N D N K T V D S K SEC2 110 C M Y G G I T K H E G N SEC2 213 Y L M M Y N D N K T V D S K SEC3 110 C M Y G G I T K H E G N SEC3 213 Y L M I Y K D N K M V D S K SED 101 C T Y G G V T P H E G N SED 204 Q L R I Y S D N K T L S T E SEE 103 C M Y G G V T L H D N N SEE 206 L L R I Y R D N K T I N S E SPEA 98 C I Y G G V T N H E G N SPEA 197 Y L M I Y K D M E T L D S N SPEC 85 Y I Y G G I T P A Q N N SPEC 184 I F A K Y K D N R I I N M K TSST-1 83 F Q I S G V T N T E K L TSST-1 179 P P I N I D E I K T I E A E *From Hoffmann et al., Infect Immunol 62: 3396 (1994).

TABLE 4 SEC1 LOOP MUTANTS AMINO ACID # 93  94  95  96  97  98  99  100 101 102 103 104 105 106 107 108 109 110 SEC1 (wild type) AMINO ACID Cys Tyr Phe Ser Ser Lys Asp Asn Val Gly Lys Val Thr Gly Gly Lys Thr Cys NUCLEIC ACID TGC TAT TTT TCA TCC AAA GAT AAT GTA GGT AAA GTT ACA GGT GGC AAA ACT TGT SEC1 Loop Deletion                         301 -   306 Mutants  −4 A.A. MUTANT Cys Tyr Phe Ser Ser Lys Asp Asn Ala                 Gly Gly Lys Thr Cys TGC TAT TTT TCA TCC AAA GAT AAT GCA --------------- GGT GGC AAA ACT TGT  −9 A.A. MUTANT Cys Tyr Phe Ser Ser                                     Gly Lys Thr Cys TGC TAT TTT TCA TCC -------------------------------     GGC AAA ACT TGT −12 A.A MUTANT Cys Cys                                                 Gly Lys Thr Cys TGC T-- ------------------------------------------- GT  GGC AAA ACT TGT 

1-26. (Canceled)
 27. A method for reducing the toxicity of a disulfide loop containing bacterial toxin comprising: mutating the disulfide loop region to contain no more than 10 amino acid residues.
 28. The method of claim 27, wherein the bacterial toxin is a staphylococcal enterotoxin or a streptococcal toxin.
 29. The method of claim 28, wherein the staphylococcal toxin is a type A, B, C, D, E, G, or H staphylococcal enterotoxin.
 30. The method of claim 28, wherein the streptococcal toxin is streptococcal pyrogenic exotoxin A or streptococcal superantigen.
 31. The method of claim 27, wherein the disulfide loop region is modified to contain no more than 6 amino acid residues.
 32. The method of claim 28, wherein the staphylococcal toxin is a type C staphylococcal enterotoxin.
 33. The method of claim 27, wherein at least 8 amino acids are deleted from within the disulfide loop region.
 34. The method of claim 27, wherein at least 12 amino acids are deleted from within the disulfide loop region.
 35. The method of claim 32 wherein the type C staphylococcal enterotoxin is staphylococcal enterotoxin C1.
 36. The method of claim 32 wherein the staphylococcal enterotoxin is staphylococcal enterotoxin C1, staphylococcal enterotoxin C2, staphylococcal enterotoxin C3, staphylococcal enterotoxin C-MNCopeland, staphylococcal enterotoxin C-4446, staphylococcal enterotoxin C-bovine, staphylococcal enterotoxin C-canine or staphylococcal enterotoxin C-ovine.
 37. A method for reducing the toxicity of a disulfide loop containing bacterial toxin comprising: deleting at least 40% of the residues of the disulfide loop region.
 38. A method for reducing the toxicity of a disulfide loop containing bacterial toxin comprising: mutating the region from residue 93 to residue 110 to contain no more than 10 amino acid residues; wherein the bacterial toxin is selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO:
 8. 39. A modified pyrogenic toxin derived from a native disulfide loop-containing pyrogenic toxin, wherein the modified toxin comprises a disulfide loop region containing no more than 10 amino acid residues.
 40. The modified toxin of claim 39 wherein the native disulfide loop-containing pyrogenic toxin is a staphylococcal toxin or a streptococcal toxin.
 41. The modified toxin of claim 40 wherein the staphylococcal toxin is a type A, B, C, D, E, G, or H staphylococcal enterotoxin.
 42. The modified toxin of claim 40 wherein the streptococcal toxin is streptococcal pyrogenic exotoxin A or streptococcal superantigen.
 43. The modified toxin of claim 39 wherein the disulfide loop region contains no more than 6 amino acid residues.
 44. The modified toxin of claim 39 wherein the native disulfide loop-containing pyrogenic toxin is a type C staphylococcal enterotoxin.
 45. The modified toxin of claim 44 wherein the modification comprises a deletion of at least 8 amino acid residues within the disulfide loop region.
 46. The modified toxin of claim 44 wherein the modification comprises deletion of at least 12 amino acid residues within the disulfide loop region.
 47. The modified toxin of claim 44 wherein the staphylococcal enterotoxin is staphylococcal enterotoxin C2, staphylococcal enterotoxin C3, staphylococcal enterotoxin C-MNCopeland, staphylococcal enterotoxin C-4446, staphylococcal enterotoxin C-bovine, staphylococcal enterotoxin C-canine or staphylococcal enterotoxin C-ovine.
 48. The modified toxin of claim 39 comprising a cysteine residue at position 2 of the disulfide loop region.
 49. The modified toxin of claim 39 having substantially decreased toxicity in comparison to the native toxin.
 50. The modified toxin of claim 49 having an emetic response inducing activity decreased by at least 100-fold in comparison to the native toxin.
 51. The modified toxin of claim 49 having an fever inducing activity decreased by at least 100-fold in comparison to the native toxin.
 52. The modified toxin of claim 49 having an LD₅₀ in Dutch Belted or New Zealand White rabbits which is at least 100-fold higher than the native toxin.
 53. A modified pyrogenic toxin derived from a native disulfide loop-containing pyrogenic toxin, wherein the modification comprises deletion of at least 40% of the amino acid residues within the disulfide loop of the native toxin.
 54. The modified pyrogenic toxin of claim 53 comprising a cysteine residue at position 2 of the disulfide loop region.
 55. The modified pyrogenic toxin of claim 53 wherein the native pyrogenic toxin is a type C staphylococcal enterotoxin.
 56. An isolated nucleic acid comprising a nucleotide sequence encoding a modified pyrogenic toxin derived from a native disulfide loop-containing pyrogenic toxin, wherein the modified toxin comprises a disulfide loop region which includes no more than 10 amino acid residues.
 57. An isolated nucleic acid comprising a nucleotide sequence encoding a modified staphylococcal enterotoxin derived from a native staphylococcal enterotoxin, wherein the modification comprises deletion of at least 40% of the amino acid residues within the disulfide loop region of the native enterotoxin.
 58. A modified pyrogenic toxin derived from a native type C staphylococcal enterotoxin, wherein the modified toxin comprises a disulfide loop region which includes no more than 10 amino acid residues. 